MINOR: stats/debug: maintain a counter of debug commands issued

Debug commands will usually mark the fate of the process. We'd rather
have them counted and visible in a core or in stats output than trying
to guess how a flag combination could happen. The counter is only
incremented when the command is about to be issued however, so that
failed attempts are ignored.
diff --git a/include/types/stats.h b/include/types/stats.h
index 8817746..52ea18d 100644
--- a/include/types/stats.h
+++ b/include/types/stats.h
@@ -327,6 +327,7 @@
 	INF_FAILED_RESOLUTIONS,
 	INF_TOTAL_BYTES_OUT,
 	INF_BYTES_OUT_RATE,
+	INF_DEBUG_COMMANDS_ISSUED,
 
 	/* must always be the last one */
 	INF_TOTAL_FIELDS